FIELD OF THE INVENTION
The invention relates to a switched-mode power supply having a rectifier configuration which supplies a rectified voltage at its output, a transformer with a primary winding and a secondary winding for connecting a load, a switching transistor for the clocked application of the rectified voltage to the primary winding, and a drive unit which controls the switching transistor.
In switched-mode power supplies the current flowing through the primary winding of the switched-mode power supply transformer is chopped by means of the switching transistor. The voltage pulses induced on the secondary side of the transformer are rectified and smoothed in order to supply a load with the D.C. voltage. The turn-on and turn-off phases of the switching transistor are controlled in dependence on the loading in such a way that the secondary voltage is regulated to be constant and as far as possible independent of load.
A power supply of the generic type is disclosed, for example, in my earlier, commonly assigned U.S. Pat. No. 5,420,776. The control unit there contains an integrated circuit designated TDA 4605 which, for the purpose of supplying the operating voltage, is connected via voltage dividers with a bridge rectifier. The latter rectifies the input A.C. voltage. The output of the control unit is connected to the control terminal of the switching transistor which is coupled with the primary winding. The withstand voltage of the control unit must thereby be chosen in dependence on the strength input A.C. voltage.
FIG. 3 of published PCT application Wo 97/50165 discloses a switched-mode power supply which has a rectifier configuration at the output of which a rectified voltage is available which, by means of a switching transistor, is applied to a primary winding belonging to a transformer and connected in series with the switching transistor. A drive circuit of the switching transistor is coupled to the secondary coil in that prior art switched-mode power supply, and is connected to a control terminal of the switching transistor by means of a transformer.
Published PCT application WO 98/03136 discloses a switched-mode power supply in which a D.C. voltage which is available is applied by means of a switching transistor to a primary winding belonging to a transformer and connected in series with the switching transformer. In order to drive the switching transistor, a pulse generator is provided, which is coupled to the control connection of the switching transistor by means of a transformer. In that case, the pulse generator is connected to a secondary coil of the transformer. In order to permit the switched-mode power supply to be started up, a start circuit configuration is provided on the primary side, which picks up energy from the D.C. voltage that is present on the primary side and which is connected to the control terminal of the switching transistor.